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Input Meteosat images

To compute the HelioClim-3 solar radiation database in real time, our two sites (MINES ParisTech - Transvalor) have been supplied with: reception dishes, acquisition chain, computational resources and servers able to generate HelioClim-3 and to fulfill the requests from the users. The two chains are synchronized for an enhanced robustness of the SoDa Service. The model Heliosat-2 processes the images of the third satellite of the Meteosat Second Generation (MSG-3) collected between 3:00 UT to 20:45 UT (72 images) to compute the 15 minute Global Irradiance values over a horizontal plane (GHI).

What is actually stored in HC3?

The HelioClim-3 database stores the 15 minuteGHI values. When the user launch a request, post-processing layers are applied to correct these values, as illustrated by the following picture on HelioClim-3 version 5. Then decomposition models computes the radiation components on the plane orientation as requested.

Missing values in HC3: Several slots (or images) are regularly missed, due to internal shortage or interruption of service by Eumetsat. Except is the whole images of a day are missing, this is totally invisible for the users since the images of a day are interpolated if at least one image is available. For you information, the missing slots are available on this page with the corresponding information, if available (missing value in HelioClim: -999). Please note that the field of view is restricted to the pixels belonging to the Earth and for which the elevation angle of the sun above horizon is larger than 12 degrees.

Hypothesis "end of time integration": the date or instant given for each value corresponds to the end of the summarization/time integration. I.e. 24/04/2010 08:00 means that the associated irradiation or irradiance value has been measured between 07:00 and 08:00. This convention addresses the standard recommanded by the WMO (World Meteorological Organisation).

Outputs and derived services

From this database, several types of services are proposed: time series of radiation values (via an annual subscription or on-request), maps of radiation (on-request), typical meteorological years (on-request), calibration of HC3 data using ground measurements (on-request)...

The FREE version of the HC3 Web service covers Feb. 2004 till end 2006. The HC3 Web service for pay which delivers data for more recent years is accessible via different types of annual subscriptions. The price of the annual subscription depends on several parameters (time step of the data, type of access, plane orientation...), don't hesitate to contact the SoDa support to receive a quotation.

HelioClim-3 and Digital Elevation Model

Topography impacts the solar radiation values at ground levels in two ways: the variation of the optical path length and the shadowing effect due to the horizon. Indeed, according to the atmospheric layer tickness crossed by sun rays when reaching the ground, the radiation is modulated: the higher the elevation of a point, the narrower the atmospheric optical layer, and finally the higher the radiation. Each HelioClim-3 value is stored using the default elevation database TerrainBase 5' (worlwide database, spatial resolution of 5' of arc, equivalent to approx. 10 km) to be checked.An altitude correction is applied "on the fly" to take into account the exact height of the point requested by the user, using the SRTM database (Shuttle Radar Topography Mission, available worldwide, 90 m of spatial resolution and 10 m of uncertainty). The user just has to set the altitude to the default "Automatic" option. It is also possible to take into account the shadowing effect due to the far horizon by putting the "Compute Relief Shadows" input option to "TRUE".

Content: In this publication, Cano et al. recommended to apply the Heliosat-2 method only for the slots when the sun is above 18° of elevation. We decided to slightly descrease this threshold to 12° in our implementation of Heliosat-2 (HelioClim-3).